1. Field of the Invention
The present invention relates to an apparatus for driving a light emitting element and a system for driving light emitting elements, which drive a light emitting element by causing a direct current to flow thereto, and in particular the invention relates to a preferred apparatus for driving a light emitting element and a preferred system for driving the same, which are used to drive a light emitting element having a large interval resistance (series resistance), represented by a surface emission type laser element.
2. Description of the Related Art
High resolution and high speed have been demanded in the field of laser xerography using a laser beam as a light source. There is a limitation in the speed (hereinafter called “modulation speed”) for controlling ON and OFF of the drive of a laser element in response to input image data. If an attempt is made to raise not only the resolution power in the main scanning direction but also the resolution power in the subscanning direction where the number of laser beams is singular, the modulation speed will be more or less sacrificed. Therefore, in order to raise the resolution power in the subscanning direction without raising the modulation speed, there is no way other than the number of laser beams is increased. Where it is assumed that the modulation speed is identical to that in the case where the number of laser beam is singular if the number of laser beams is made, for example, four, the resolution power in the main scanning direction and subscanning direction may be increased double.
However, the semiconductor laser is roughly classified into an edge emitting type laser element (hereinafter called an “edge emitting laser”) having such a structure in which a laser beam is picked up in a direction parallel to the active layer, and a surface emission type laser element (hereinafter called a “surface emitting laser”) having such a structure in which a laser beam is picked up in a direction perpendicular to the active layer. Conventionally, the edge emitting laser has generally been used as a laser light source in laser xerography.
However, in view of increasing the number of laser beams, the edge emitting laser has a technical difficulty. A surface emitting laser is more advantageous to increase the number of laser beams than the edge emitting laser in terms of structure. Based on such a reason, recently, an apparatus employing, as a laser light source, a surface emitting laser that is able to emit a number of laser light beams has been advanced in order to meet higher resolution power and higher speed in laser xerography.
Herein, a description is given of a laser driving apparatus that has been used for laser xerography. The laser driving apparatus is roughly classified into three types, one of which is a voltage drive type, another of which is a current output/voltage drive type, and the other of which is a current drive type. Hereinafter, a description is given of the respective types of laser driving apparatus.
First, such a structure that controls voltage applied to a laser element directly at the drive circuit side has been known as to the voltage drive type laser driving apparatus (For example, refer to Japanese Unexamined Patent Publication No. 1999-68198). Since the laser driving apparatus is devised so as to control the optical power by directly controlling the power source voltage of a logic gate, it can be constructed at a remarkably cheap cost.
Next, as to the current output/voltage drive type laser driving apparatus, such a structure has been known, in which the current source and laser element are connected to each other in series, and a drive voltage is generated in the vicinity of the laser element by using a termination resistor that is connected in parallel to the laser element (For example, refer to Japanese Unexamined Patent Publication No. 1984-18964). In the case of the laser driving apparatus, an output is a current flowing through the current source, and it is comparatively easy to generate an optional current in comparison with generation of a number of voltages having low output impedance.
Finally, as to the current drive type laser driving apparatus, such a structure has been known, in which a current generated by a constant current circuit is controlled so as to be turned on and off by a current switch and is provided into a laser element (For example, refer to Japanese Unexamined Patent Publication No. 1983-13790). Conventionally, with regard to driving of the edge emitting laser, this type of current drive type laser driving apparatus has been generally employed. The reasons are as follows;
As shown in FIG. 26, since the drive current is exponential-functionally increased with respect to an application voltage in the edge emitting laser, a differential resistance (ΔV/ΔI) fluctuates, depending on a bias point if the drive current is controlled by a voltage, wherein since nonlinear elements are generated in a negative feedback loop for control, the control becomes difficult. To the contrary, if the edge emitting laser is driven by a current, since the optical power is proportionate to the current over the threshold current of laser oscillation, the negative feedback loop is constituted with linear elements, wherein the control can be facilitated. Further, even in the case of driving a number of laser elements, it is possible to comparatively easily provide current sources for respective laser elements if the electric drive is adopted.
Herein, a description is given of a difference in the electrical aspect in view of drive between a surface emitting laser, which can be used for laser xerography, and a conventional edge emitting laser. The difference resides, as shown in FIG. 26, in that, although a current exponential-functionally increases to 100 mA or so with respect to an application voltage onto a laser element in a conventional edge emitting laser, the current is very small like several hundreds of microampere in the edge emitting laser, and the voltage-current characteristics may become linear.
The reason is as follows; That is, wherein the edge emitting laser is used for xerography, it is necessary to cause the light emitting laser to emit light in a single mode so that the laser light does not diffuse. Therefore, the area of light emission should be throttled. If the area of light emission is throttled, the area of conjunction may be narrowed, thereby resulting in an increase in the value of internal resistance in an equivalent circuit of a surface emitting laser, which is shown in FIG. 27. Accordingly, with only a flow of small current, the voltage-current characteristics may become linear.
On the other hand, if a current is increased in the case of an edge emitting laser, the voltage-current characteristics finally become linear due to a cause of the internal resistance. However, the current value entering the linear area is greater by one or more digits than that of the surface emitting laser. That is, in the equivalent circuit shown in FIG. 28, the value of internal resistance becomes several hundreds of ohms (Ω) in the surface emitting lasers while the value of the internal resistance is several tens of ohms (Ω) in the edge emitting laser. The former becomes greater by one or more digits than the latter.
Where the surface emitting laser is further provided with a number of light emission parts that emit a number of laser beams in order to meet a request of higher resolution power and higher speed in laser xerography, the drive apparatus tends to become large in size since it must drive a number of light emission parts. Therefore, as shown in FIG. 29, the distance of routed wires is lengthened. Also, as has been made clear in FIG. 29, a number of routed wires are arranged in parallel to each other, wherein the parasitic capacitance will be increased to cause crosstalk to be liable to occur due to between-wire capacitance and common impedance.
In view of the modulation speed, in the case of the edge emitting laser, the value of internal resistance is small (See FIG. 28), and the length of the routed wires is short as shown in FIG. 30, wherein the parasitic capacitance is small. As a result, since a time constant τ that is determined by a resistance value R of the internal resistance and capacitance C of the parasitic capacitance is small, rise and fall of the drive current waveform are made steep as shown in FIG. 32. On the other hand, in the case of an edge emitting laser, as described above, the value of the internal resistance is large (See FIG. 27), and the wiring length is long, wherein the capacitance including parasitic capacitance with adjacent wires is made large, and the time constant τ becomes large. Accordingly, as shown in FIG. 31, the rise and fall of the drive current waveform become remarkably slow.
In the current drive type laser driving apparatus according to a prior art example, which has been described above, only 1 nsec is sufficient to start the edge emitting laser. To the contrary, in the case of the surface emitting laser, the time constant increases several tens of times in comparison with the time constant of the edge emitting laser, wherein the modulation speed is several tens of MHz. This implies that the modulation speed does not rise as a whole although the number of emitting laser beams is multiple. Therefore, unless the modulation speed is remarkably improved, there is no advantage in using the surface emitting laser as the laser light source in laser xerography.
In the above-described viewpoint, in order to drive a surface emitting laser that is able to emit a number of laser beams, the voltage drive type laser driving apparatus is more advantageous than the current drive type laser driving apparatus. That is, where it is assumed that the current drive type driving apparatus has an idealistic current source and the voltage drive type driving apparatus has an idealistic voltage, it is considered that these have an infinite resistance and a zero resistance value Ro in parallel to stray capacitance C at the drive end of a light emitting element having an internal resistance value Ri, which is a subject to be driven, with respect thereto. Therefore, a resistance component R of a time constant CR by which the speed of rise and fall can be regarded as parallel synthetic resistance of Ro and Ri, wherein the former is mainly the internal resistance of a light emitting element while the latter is mainly a resistance value at the driving apparatus side. Also, in the case of the current output/voltage drive type laser driving apparatus, there is a system in which a current output is caused to flow into a resistor connected to a laser element in parallel, and the laser element is driven by its voltage drop. However, it is necessary to lower the resistance value of a parallel resistor in order to raise the modulation speed, wherein consumption power will be accordingly increased remarkably.
Here, the voltage drive type laser driving apparatus described in Japanese Unexamined Patent Publication No. 1999-68198 is taken into consideration again. The voltage drive type laser driving apparatus according to the prior art example employs a CMOS logic gate 101 as shown in FIG. 33. Two potentials of the ground level and power source voltage are changed over, and the voltage is applied to a laser element 103 via a resistor 102. At the same time, the back light outputted from the laser element 103 is received by a photo diode 104, and the power source voltage of the logic gate 101 is directly controlled on the basis of the received optical power via a feedback circuit 105, whereby the optical power can be automatically controlled so that the laser element 103 emits light at an appointed optical power. Also, the feedback circuit 105 is provided with a voltage source 106 to control the optical power.
However, in the voltage drive type laser driving apparatus thus constructed according to the prior art example, controllability is secured by providing a resistor 102 between the logic gate 101 and the laser element 103 and substantially driving laser elements with a current. Therefore, where the surface emitting laser is driven in the voltage drive type laser driving apparatus according to the prior art example, the resistor 102 that intervenes between the logic gate and the surface emitting laser becomes a factor of suppressing the modulation speed, which hinders an increase in the modulation speed.
Also, where a surface emitting laser that is a subject to be driven is applied to laser xerography, it is necessary that automatic light control is carried out for each of a number of light emission parts. Therefore, it is necessary to individually prepare a voltage source 106 for all of the logic gates 106 for drive. Further, since the power source of the logic gates is usually common, it is impossible to individually control a laser (light emission parts) by a single IC including a plurality of gates.
Also, at this time, performance requested for the voltage source 106 is such that the output impedance is low. Therefore, it is necessary to prepare a countermeasure of generally increasing a bias current in order to provide a decoupling capacitor of power source output in an IC chip and to lower the output impedance of the power source circuit. However, if a decoupling capacitor is provided, or a countermeasure of increasing the bias current is taken, it becomes a large limitation in designing IC chips in the view point of mounting and consumption power.
The present invention was developed in view of the above-described and other situations, and it is therefore an object of the invention to provide an apparatus for driving a light emitting element and a system for driving light emitting elements that is able to drive light emitting elements such as surface emitting lasers, etc., and enable higher modulation speed without any increased in the consumption power and without any limitation in the circuit integration (CI).